[mcabbott]

Yes, although that seems like the easy half of this, making sure `struct NewNum <: AbstractFloat` defines everything. There aren't yet tools for this but they are easy to imagine. And missing methods do give errors.

The hard half seems to be correctness of functions which accept quite generic objects. For example writing `f(x::Number)` in order to allow units, means you also allow quaternions, but many functions doing that will incorrectly assume numbers commute. (And not caring is, for 99% of these, the intention. But it's not encoded anywhere.) Less obviously, we can differentiate many things by passing dual numbers through `f(x::Real)`, but this tends to find edge cases nobody thought of. Right now if your algorithm branches on `if det(X) == 0` (or say a check that X is upper triangular) then it will sometimes give wrong answers. This one should be fixed soon, but I am sure there are other subtleties.